Dominique McKay

For the simple cost of transit, tourists flock year-round to the National Mall to view historical artifacts - from presidential portraits to the legendary Hope Diamond - making the free Smithsonian museums among the District's most popular attractions.

But private museums charging admission have made some headway, including the International Spy Museum, which opened in July 2002 and charges $19.95 for adult general admission, and the Newseum, which opened at its current location in April 2008 and charges $21.95 for adult general admission.